Calculations

How do you calculate the relative formula mass of a compound?

Add together all the relative atomic masses of the atoms in the compound.

What is the empirical formula?

The empirical formula is the smallest whole number ratio of the atoms of each element in a compound.

What 2 values could be used to calculate the empirical formula of a simple compound?

Reacting masses or percentage composition can be used to calculate the empirical formula.

What is the empirical formula for Fe2O4?

FeO2

What is the molecular formula?

Actual number of atoms of each element in a compound.

Describe an experiment to work out the empirical formula of magnesium oxide?

Weigh a sample of magnesium.

Heat the sample in a crucible.

Weigh the mass of magnesium oxide at the end.

Calculate the mass of oxygen (this is the increase of mass).

Calculate the moles of magnesium and oxygen using the experimental mass and relative atomic mass.

Work out the whole number ratio of the number of moles of magnesium to oxygen.

What is the law of conservation of mass?

No matter is lost or gained during a chemical reaction.

If a reaction is carried out in a closed system, what can you say about the total mass of the reaction throughout the experiment?

Mass stays constant.

If a reaction is carried out in an open flask and a gas is produced, what can you say about the total mass of the reaction throughout the experiment?

Mass decreases as the gas escapes.

52g of calcium reacts with oxygen to form 79g of calcium oxide. Using the law of conservation of mass, what mass of oxygen is needed?

79 − 52 = 27. Mass of oxygen = 27g.

What equation links mass, moles and relative atomic mass? (higher only)

Mass (g) = Moles × Relative atomic mass (Mr).

How can you calculate concentration in g/dm3?

Concentration (g/dm3) = Mass (g) / Volume (dm3).

What is the Avogadro constant? (higher only)

The number of atoms, molecules or ions in one mole of a given substance. The value of the constant is 6.02 × 10²³.

What is the mass of 20 moles of calcium carbonate, CaCO3? (higher only)

Mass (g) = Moles × Relative atomic mass (Mr). Mr = 100. 20 × 100 = 2000 g.

What formula links the Avogadro constant, moles and number of particles? (higher only)

Number of particles = Avogadro constant × Moles.

How many atoms are in 3 moles of copper? (higher only)

Number of atoms = Avogadro’s constant × Moles = 6.02 × 10²³ × 3 = 1.81 × 10²⁴.

What is a limiting reagent in a chemical reaction? (higher only)

The chemical that is used up first in a reaction, preventing the formation of more product.

Typically, an excess of one of the reactants is used to ensure that the other reactant is completely used up.